Analysis of CNS sympatho-inhibition produced by guanabenz

Novel strategies and targets for the management of hypertension

Hypertension, as the sole or comorbid component of a constellation of disorders of the cardiovascular (CV) system, is present in over 90% of all patients with CV disease and affects nearly 74 million individuals in the United States. The number of medications available to treat hypertension has dramatically increased during the past 3 decades to some 50 medications as new targets involved in the normal regulation of blood pressure have been identified, resulting in the development of new agents in those classes with improved therapeutic profiles (e.g., renin-angiotensin-aldosterone system; RAAS). Despite these new agents, hypertension is not adequately managed in approximately 30% of patients, who are compliant with prescriptive therapeutics, suggesting that new agents and/or strategies to manage hypertension are still needed. Some of the newest classes of agents have targeted other components of the RAS, for example, the selective renin inhibitors, but recent advances in vascular biology have provided novel potential targets that may provide avenues for new agent development. These newer targets include downstream signaling participants in pathways involved in contraction, growth, hypertrophy, and relaxation. However, perhaps the most unique approach to the management of hypertension is a shift in strategy of using existing agents with respect to the time of day at which the agent is taken. This new strategy, termed "chronotherapy," has shown considerable promise in effectively managing hypertensive patients. Therefore, there remains great potential for future development of safe and effective agents and strategies to manage a disorder of the CV system of epidemic proportion.

Rilmenidine produces mydriasis in cats by stimulation of CNS alpha2-adrenoceptors

1. Experiments were undertaken to determine if the imidazoline/alpha2-adrenoceptor agonist, rilmenidine, would produce mydriasis in cats and, if so, to delineate its site of action and determine if this effect is mediated by imidazoline receptors or alpha2-adrenoceptors. 2. Rilmenidine produced dose-related pupillary dilator responses in pentobarbital anaesthetized cats that were independent of sympathetic innervation to the iris but were dependent upon intact parasympathetic neuronal tone. The ED50 for rilmenidine-induced pupillary dilation was approximately 200 microg kg(-1), i.v., and was sustained for at least 1 h. 3. The highly selective alpha2-adrenoceptor antagonist, RS-79948, administered either before or after rilmenidine, antagonized rilmenidine-induced mydriasis. Neuronally induced reflex inhibition of parasympathetic nerve activity was also inhibited by administration of RS-79948. 4. These results suggest that rilmenidine acts like clonidine to produce pupillary dilation by inhibition of parasympathetic tone to the iris sphincter and that this central nervous system parasympatho-inhibition is mediated by alpha2-adrenoceptors, rather than imidazoline receptors.

Effect of prazosin on spontaneous sympathetic-cholinergic activity

Tonic sympathetic-cholinergic electrodermal (sudomotor) activity was measured in intact anesthetized and unanesthetized decerebrate and decerebrate-spinalized cats. Prazosin (3-100 micrograms/kg i.v.) depressed spontaneous electrodermal activity in intact anesthetized cats in a dose-dependent fashion (ED50 4.8 micrograms/kg). Prazosin's action was almost totally abolished by monoamine depletion with reserpine and a synthesis inhibitor. The alpha 2-adrenoceptor blocker, yohimbine (0.5 mg/kg i.v.) also antagonized prazosin-induced sympatho-inhibition. Prazosin reduced tonic sudomotor activity in unanesthetized decerebrate cats in a dose-dependent fashion (ED50 5.5 micrograms/kg i.v.) but was without effect in spinalized preparations. These results support the hypothesis that, in this system, prazosin produces sympatho-inhibition indirectly by means of an alpha 2-adrenoceptor-mediated mechanism, an effect which appears to be prejunctional. It is proposed that prazosin acts at the level of the spinal cord to facilitate ongoing alpha 2-adrenergic inhibition arising from supra-spinal loci.

Mechanism of dexamphetamine-induced mydriasis in the anaesthetized rat

1. The effect of intravenous administration of dexamphetamine [+)-Amp) on rat pupil diameter was investigated. In all experiments, the vagosympathetic trunks were sectioned bilaterally at the cervical level. 2. In rats anaesthetized with urethane, (+)-Amp (0.1-0.3 mg kg-1, i.v.) produced a dose-related increase in pupil size. The mydriatic effects of (+)-Amp were evident immediately after administration. 3. Pretreatment with the alpha 2-adrenoceptor antagonists yohimbine (1.5 mg kg-1 i.v.) or idazoxan (0.5 mg kg-1, i.v.) blocked the pupillary response to (+)-Amp. Yohimbine caused about a 30 fold shift to the right in the dose-response curve whereas idazoxan almost completely abolished the mydriatic response to (+)-Amp. 4. In contrast, pretreatment with the alpha 1-adrenoceptor antagonist phenoxybenzamine (2 mg kg-1, i.v.), failed to alter significantly the pupillary response to (+)-Amp. 5. Depletion of central nervous system (CNS) monoamines with reserpine (5 mg kg-1, i.p.) and alpha-methyl-p-tyrosine (2 x 300 mg kg-1, i.p.) prevented the pupillary response to (+)-Amp. 6. The mydriatic effect of (+)-Amp was present only in preparations that had intact parasympathetic neural tone to the iris. Central preganglionic denervation of the oculomotor nerve abolished the mydriatic response of (+)-Amp. 7. These results indicate the (+)-Amp acts in the CNS to produce mydriasis in the anaesthetized rat by stimulating CNS postsynaptic alpha 2-adrenoceptors, findings that are consistent with the hypothesis that (+)-Amp acts predominantly as an indirect sympathomimetic agent to release endogenous stores of a monoaminergic neurotransmitter (perhaps noradrenaline).

Characterization of alpha-adrenoceptors in the nucleus reticularis gigantocellularis involved in the cardiovascular depressant effects of guanabenz in the rat

The participation of alpha-adrenoceptors in the nucleus reticularis gigantocellularis in the hypotensive, negative inotropic and chronotropic effects induced by guanabenz, was examined in rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Pretreatment with alpha-adrenoceptor antagonists yohimbine (10 micrograms), phentolamine (2.5 micrograms) and phenoxybenzamine (20 micrograms), which were injected bilaterally into the nucleus reticularis gigantocellularis, significantly antagonized the cardiovascular suppressant effects normally produced by systemic administration of guanabenz (10 micrograms/kg, i.v.). Pretreatment with prazosin (0.25 microgram) did not affect the vasodepressive, but significantly attenuated the bradycardic actions of guanabenz. The general trend of "antagonization potency" shown by the alpha-adrenergic blockers, against the cardiovascular effects of guanabenz, was in the order: yohimbine greater than phentolamine greater than phenoxybenzamine greater than prazosin. It is concluded that while the alpha 2-adrenoceptors in the nucleus reticularis gigantocellularis are more critically involved in the antihypertensive actions of guanabenz, the possibility exists that alpha 1-adrenoceptors may also participate, in part.

Correlated effects of guanabenz on single-neuron activity in the nucleus reticularis gigantocellularis, systemic arterial pressure and heart rate in the rat

The simultaneous effects of guanabenz on the activity of single-neurons in the nucleus reticularis gigantocellularis of the medulla oblongata, systemic arterial pressure and heart rate were assessed in Sprague-Dawley rats, anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Of the 35 arterial pressure-related neurons (neurons that temporally altered their spike rhythm subsequent to fluctuations in arterial pressure) which were evaluated in this reticular nucleus, 32 changed their discharge frequencies that exhibited a degree and time-course parallel to the hypotension promoted by the amino-guanidine compound (5, 10 or 20 micrograms/kg, i.v.). More importantly, these alterations neuronal activity preceded the occurrence of the induced vasodepression, signifying a causative relationship between the two events. Four of the 5 non-arterial pressure-related neurons in the gigantocellular reticular nucleus, on the other hand, manifested no basic modification in their spike frequencies in relation to the hypotension induced by guanabenz. These observations provided further support for the idea that the nucleus reticularis gigantocellularis participates actively in the cardiovascular suppressive actions of the aminoguanidine compound.

Effects of adrenoceptor agonists and antagonists on sulfobromophthalein disposition in mice

The effects of alpha 2-adrenoceptor agonists on sulfobromophthalein (BSP) disposition in mice were studied. It was found that agents with both central and peripheral activities (clonidine, guanabenz, B-HT 920 and methyldopa) as well as peripherally acting alpha 2-adrenoceptor agonists (para amino-clonidine and St 91) inhibited sulfobromophthalein disposition in the mouse, and also caused substantial hypothermia. The effects of these agonists were inhibited by yohimbine, a specific alpha 2-antagonist, except for those of para amino-clonidine where only partial reversal was achieved. The effects of clonidine on BSP disposition were also reversed by piperoxan but not by the alpha 1-adrenoceptor antagonists, prazosin and phenoxybenzamine, nor by the beta-blocker, propranolol. These results suggest that, in mice, peripheral alpha 2-adrenoceptors are involved in the effects of the alpha-agonists on BSP disposition.

Interactions between guanabenz and clonidine in their antinociceptive effects in the rat

Based on the hot-plate algesiometric assay in conscious Sprague-Dawley rats, we observed that both guanabenz and clonidine, at a subcutaneous dose of 5 and 1 mg/kg, respectively, exhibited significant antinociceptive potencies. Paradoxical actions on the hot-plate responses, however, were exerted by guanabenz at lower doses, varying from hyperalgesia (1 mg/kg) to no effect (2 mg/kg). A pretreatment of clonidine (1 mg/kg) substantially potentiated the antinociceptive efficacy of the two latter doses, but failed to further augment the pain-suppressive action of guanabenz at 5 mg/kg. These interactive effects of the two alpha-adrenoceptor agonists suggest that they may share a common mechanism(s) in their analgesic actions.

Pupillary dilation as an index of central nervous system alpha 2-adrenoceptor activation

In recent years there has been increasing evidence that some antihypertensive drugs like clonidine and alpha-methyldopa (after conversion in the brain to alpha-methylnorepinephrine) may decrease sympathetic tone by stimulating central nervous system (CNS) alpha 2-adrenoceptors. These same drugs also produce pupillary dilation in cats and rats. In this review, evidence is presented supporting the hypothesis that clonidinelike drugs act either directly or indirectly on CNS postsynaptic alpha 2-adrenoceptors to cause pupillary dilation by reduction of parasympathetic neural tone to the iris. It is further suggested that the underlying physiologic mechanism for this mydriatic action is activation of an ascending pathway that provides tonic inhibitory input by releasing norepinephrine on neurons in the Edinger-Westphal complex. Yohimbine-sensitive pupillary dilation in these species may provide a simple and effective model for quantitatively accessing CNS alpha 2-adrenoceptor activity.